Re: gEDA-user: Exposed copper -- still need to make a pad?

[John Doty <jpd@xxxxxxxxxxxxx>]

On Sep 27, 2007, at 10:06 AM, Randall Nortman wrote:

> On Thu, Sep 27, 2007 at 09:44:36AM -0600, John Doty wrote:
>>
>> On Sep 27, 2007, at 9:28 AM, Randall Nortman wrote:
>>
>>> I need to make an exposed area of copper (no soldermask), to act  
>>> as a
>>> heat radiator.
>>
>> Why no soldermask? I'll bet its infrared emissivity is better than
>> copper's, making a better radiator. In space, the radiator tape we
>> use has a thin layer of teflon over the metal for this reason.
>
> As sayeth the datasheet, so do-eth the designer!

As sayeth the physics, so doeth this designer! Datasheets are  
extremely valuable, but understanding what's going on behind the  
curtain is also important. Datasheets aren't the last word in a real  
application.

>   In fact, any exposed
> copper is going to be covered in shiny silver stuff (tin/lead/silver,
> depending on process) during board fab.  Would that not have pretty
> darned good emissivity?

Nope. Shiny means low emissivity. Black means high emissivity. Most  
plastics are pretty black in the infrared, but most metals are even  
shinier there than in the visible.

>   It would certainly have better conductivity,
> which is important when you're not designing for vacuum.

Conductivity is important when you're spreading the heat out. But a  
thin film of plastic over the metal is very little barrier to heat  
flow in the thin direction. A reflective surface is a real barrier to  
radiation, however. That's why we put aluminum over the plastic film  
to make a blanket.

>   I suspect
> that most of my cooling is going to be from conduction to air and
> subsequent convection (natural convection; no fan), though a big chunk
> of it will be radiation.

Won't get a lot of radiation from bare metal.

>   (I did say "heat radiator" in my post --
> probably "heat sink" would have been more appropriate.)
>
> -- 
> Randall
>
>
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John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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